Well, you either spend the noted amount of money (whether repro/Aurora Design/aftermarket) or you seek to make yours work better. It was known, back then, that the optimum length of the antenna for FM reception was 31", which is not nearly the total length it'll extend to.
You can check the resistance of the coax lead-in cables and make sure the connections are all clean, as is the external antenna sections.
Now, just for grins, look to see if there is an "RCA" plug "cap" on the backside of your factory AM/FM radio. Should be black, smaller than about a dime, and possibly held in with a small screw/bracket? IF you find one, all you need is a rear fader and "the box" to make it a factory multiplex/stereo unit. Not all had them, but some did. OF at least some GM cars came that way as Delco probably built that unit to Chrysler specs. I have run across some normal-looking Delco radios from that era that had the RCA plug, but otherwise looked like a normal 'upgrade" AM/FM radio. BUT it will take the control box, which has a thick multi-conductor cable that goes to behind the rear seat, where it is mounted, to make it work. The fader is external of the radio, but is configured for the multiplex unit. Of course, then you'll need the speakers for it, too, front and rear.
It might be possible that the superhetreodyune (or spelled similarly) tuner "plates" could be dirty and just need some spray cleaner to freshen them up? Might also be that the speakers need attention? There's a thread which mentions a vendor for factory speakers in here, somewhere.
The range on FM will be much shorter than it will be for AM, just the nature of things. FM is "line of sight" and AM can bounce off of atmospheric layers (or something to that affect). We've gotten used to the 100KW FM stations of modern times. Few existed at that power in the later '60s, so they were generally "metro area only" in reception area.
I'd suggest that you get what you've got cleaned-up and checked (the complete radio and such). Then you'll know if it's as good as you want it to be, to justify further expenditures in that area.
As mentioned, getting some modern "guts" in your existing radio would be a neat upgrade, but you'll still need the speakers and such.
Just some thoughts,
CBODY67